1. Field of the Invention
The present invention relates to a close-up attachment lens used through mounting on a taking lens with a zoom function, and more specifically a close-up attachment lens suitably used for, for example, a professional use (broadcasting) television camera.
2. Description of the Related Art
In general, zoom lenses are used for shooting cameras such as broadcasting cameras, etc. In such a zoom lens, an attachment lens is often mounted on a front or rear side of a main lens system (taking lens body) to be capable of close-up shooting. Conventionally, in most cases, such a close-up attachment lens comprises one or two single lenses. However, it is difficult for the attachment lens with such a simple configuration to satisfy optical performance requirements in a whole zoom (magnification change) range of the taking lens body on which the attachment lens is mounted.
For example, in Japanese Unexamined Patent Publication No. Hei 5-224121, a close-up attachment lens comprising two lens groups with three lenses having a positive refractive power as a whole has been proposed. The attachment lens comprises a cemented lens including a first meniscus lens with a concave surface on an object side having a positive refractive power and a second meniscus lens with a concave surface on the object side having a negative refractive power, and a third biconvex lens having a positive refractive power in order from the object side. By the use of the attachment lens, various aberrations in a whole optical system of the taking lens body and the attachment lens, specifically a change in distortion accompanied by a magnification change, axial chromatic aberration at the tele side, coma aberration, etc. are excellently compensated, so optical performance is maintained even in the case of close-up shooting at a much shorter distance, compared to a minimum shooting distance of the taking lens body.
It is assumed that the attachment lens according to the above publication is used through mounting on a taking lens body with the following configuration. That is, the attachment lens is mounted on a zoom lens comprising a first unit having a positive refractive power, a second unit having a negative refractive power, a third unit disposed in front of an aperture stop (on the object side) and having a positive refractive power and a fourth unit disposed behind the aperture stop (on the image side) and having a positive refractive power in order from the object side. The zoom lens adopts a zoom system that a magnification change from the wide end to the tele end is carried out through moving the second unit toward an image plane, and at the same time, a shift of the image plane accompanied by the magnification change is compensated through moving the fourth unit. Also, the focus of the zoom lens is adjusted through moving the fourth unit along an optical axis. That is, the zoom lens is configured so that the fourth unit is in charge of compensating a shift of the image plane and adjusting the focus. In addition, the first unit and the third unit are fixed during changing the magnification and adjusting the focus.
Except for the zoom lens with the configuration described in the above publication, there is, for example, a zoom lens for a shooting camera with a configuration disclosed in Japanese Unexamined Patent Publication No. Hei 9-5628. The zoom lens with such a configuration comprises a first unit having a focus function, a second unit having a magnification change function, and a third unit having an imaging function. If the attachment lens described in the above publication (Japanese Unexamined Patent Publication No. Hei 5-224121) is applied to the zoom lens with such a configuration, aberrations of various kinds are too large and not suitable for practical use. The reason is that the taking lens body to which the attachment lens described in the above publication (Japanese Unexamined Patent Publication No. Hei 5-224121) is applicable is a zoom lens which compensates a shift of the image plane accompanied by a magnification change and adjusts the focus through moving the fourth unit, and the zoom lens has a completely different basic configuration from the zoom lens described in Japanese Unexamined Patent Publication No. Hei 9-5628.
Thus, the attachment lens which has been conventionally proposed is applicable to the taking lens body comprising the fourth unit in charge of compensating a shift of the image plane and adjusting the focus, so it is not suitably applicable to any other taking lens bodies with different configurations.
In view of the foregoing, it is an object of the invention to provide a close-up attachment lens suitably applicable to a taking lens comprising a lens group in charge of a focus function and a lens group in charge of a magnification change function completely separate from each other.
In a close-up attachment lens according to the invention used through mounting on a taking lens body on an object side, the taking lens body including successively from an object side: a first lens group having a focus function and a positive refractive power, a second lens group having a magnification change function and a negative refractive power, a third lens group having a function of compensating a shift of an image plane accompanied by a magnification change and a negative refractive power, and a fourth lens group having an imaging function and a positive refractive power, the close-up attachment lens comprises successively from the object side: a cemented lens having a negative refractive power as a whole, the cemented lens including a first lens with a convex surface on the object side having a positive refractive power and a second lens with a concave surface on an image side having a negative refractive power; and a third single lens having a positive refractive power. Hereinafter in the description, xe2x80x9cclose-up shootingxe2x80x9d means shooting a subject in a close range in which the taking lens body cannot cover.
When the close-up attachment lens according to the invention is mounted on the taking lens body including the first lens group for focusing, the second lens group for changing a magnification, the third lens group for compensating a shift of an image plane and the fourth lens group for forming an image, preferable performance can be delivered. The close-up attachment lens functions as a lens with a configuration having two lens groups with three lenses comprising the cemented lens having a negative refractive power and the third lens having a positive refractive power. The cemented lens includes the first lens with a convex surface on the object side having a positive refractive power and the second lens with a concave surface on the image side having a negative refractive power. The close-up attachment lens is mounted to be capable of shooting a subject in a close range in which the taking lens body cannot cover while maintaining practical optical performance.
In the close-up attachment lens according to the invention, it is more preferable that the focal length f12 of the cemented lens and the focal length f3 of the third lens satisfy a conditional expression xe2x88x926 less than f12/f3 less than xe2x88x924. In this case, specifically spherical aberration is optimized.
In the close-up attachment lens according to the invention, it is more preferable that the Abbe number vd1 of the first lens and the Abbe number vd2 of the second lens satisfy conditional expressions: vd1 greater than 60.0 and vd2 less than 50.0. In this case, specifically lateral chromatic aberration is optimized.